The objective of the proposed experiments is to describe as fully as possible changes which occur in the class of abundant messenger RNA during development in the sea urchin. Using molecular hybridization techniques we are examining the persistence of abundant maternal messenger RNA species and the time of appearance of new abundant mRNAs during embryogenesis. We are isolating DNA sequences representing individual mRNA species as recombinant DNA clones. These cloned probes are being used to determine changes in the concentration of single mRNA species during development. Using the same cloned probes, we are developing in situ hybridization methods to detect complementary mRNAs in sections of unfertilized eggs and developing embryos.